A shape-memory alloy is an alloy which, when applied with a plastic deformation at a prescribed temperature near the martensitic transformation point and then heated to a prescribed temperature above the temperature at which the alloy reversely transforms into the mother phase thereof, shows a property of recovering the original shape that the alloy has had before application of the plastic deformation. By applying a plastic deformation to a shape-memory alloy at a prescribed temperature, the crystal structure of the alloy transforms from the mother phase thereof into martensite. When the thus plastically deformed alloy is heated thereafter to a prescribed temperature above the temperature at which the alloy reversely transforms into the mother phase thereof, martensite reversely transforms into the original mother phase, thus the alloy showing the shape-memory property. This causes the plastically deformed alloy to recover the original shape thereof that the alloy has had before application of the plastic deformation.
Non-ferrous shape-memory alloys have so far been known as alloys having such a shape-memory property. Among others, nickel-titanium and copper shape-memory alloys have already been practically used. Pipe joints, clothes, medical equipment, actuators and the like are manufactured with the use of these non-ferrous shape-memory alloys. Techniques based on application of shape-memory alloys to various uses are now being actively developed.
However, non-ferrous shape-memory alloys, which are expensive, are under economic restrictions. In view of these circumstances, iron-based shape-memory alloys available at a lower cost than non-ferrous ones are being developed. Expansion of the scope of application is thus expected for iron-based shape-memory alloys in place of non-ferrous ones under economic restrictions.
In terms of the crystal structure of martensite into which an iron-based shape-memory alloy transforms from the mother phase thereof by application of a plastic deformation, iron-based shape-memory alloys may be broadly classified into a fct (abbreviation of face-centered-tetragonal), a bct (abbreviation of body-centered-tetragonal), and a hcp (abbreviation of hexagonal-closed pack).
As iron-based shape-memory alloys which transform from the mother phase thereof into a fct martensite by applying a plastic deformation, iron-palladium and iron-platinum alloys are known. These iron-based shape-memory alloys show a satisfactory shape-memory property.
As iron-based shape-memory alloys which transform from the mother phase thereof into a bct martensite (hereinafter referred to as the ".alpha.'-martensite") by applying a plastic deformation, iron-platinum and iron-nickel-cobalt-titanium alloys are known. The .alpha.'-martensite is a phase which is formed in an alloy having a high stacking fault energy, resulting in a large volumic change upon transformation. A slip deformation therefore tends to occur in the .alpha.'-martensite upon transformation, and these iron-based shape-memory alloys do not show a satisfactory shape-memory property in the as-is state. It is however known that, by making the mother phase of these iron-based shape-memory alloys have the invar effect (i.e., a phenomenon in which a thermal expansion coefficient is reduced to the minimum within a certain temperature region), a slip deformation in the .alpha.'-martensite in these alloys is inhibited, and as a result, these alloys can show a satisfactory shape-memory property.
As iron-based shape-memory alloys which transform from the mother phase thereof into a hcp martensite (hereinafter referred to as the ".epsilon.-martensite") by applying a plastic deformation, a high-manganese steel and a SUS 304 austenitic stainless steel spceified in JIS (abbreviation of Japanese Industrial Standards) are known. The .epsilon.-martensite is a phase which is formed in an alloy having a low stacking fault energy, resulting in a small volumic change upon transformation. No slip deformation therefore tends to occurs in the .epsilon.martensite upon tranformation, and these iron-based shape-memory alloys show a satisfactory shape-memory property.
As an iron-based shape-memory alloy which transforms from the mother phase thereof into the .epsilon.-martensite by applying a plastic deformation, the following alloy has been proposed:
An iron-based shape-memory alloy, disclosed in Japanese Patent Provisional Publication No. 61-201,761 dated Sept. 6, 1986, which consists essentially of:
______________________________________ Manganese from 20 to 40 wt. %, silicon from 3.5 to 8 wt. %, at least one element selected from the group consisting of: chromium up to 10 wt. %, nickel up to 10 wt. %, cobalt up to 10 wt. %, molybdenum up to 2 wt. %, carbon up to 1 wt. %, aluminum up to 1 wt. %, copper up to 1 wt. %, and the balance being iron and incidental impurities ______________________________________
(hereinafter referred to as the "prior art").
The above-mentioned iron-based shape-memory alloy of the prior art has an excellent shape-memory property. More particularly, the shape-memory property available in the prior art is as follows: A test piece having dimensions of 0.5 mm.times.1.5 mm.times.30 mm was prepared by melting the iron-based shape-memory alloy of the prior art in a high-frequency heating air furnace, then casting the molten alloy into an ingot, then holding the thus cast ingot at a temperature within the range of from 1,050.degree. to 1,250.degree. C. for an hour, and then hot-rolling the thus heated ingot. Subsequently, a plastic deformation was applied to the thus prepared test piece by bending same to an angle of 45.degree. at a room temperature, and the test piece was heated to a prescribed temperature above the austenitic transformation point. Thus a shape recovering rate of the alloy was investigated: the alloy showed a shape recovering rate of from 75 to 90%.
The prior art discloses the addition of at least one element of chromium, nickle, cobalt and molybdenum to the alloy for the purpose of improving a corrosion resistance of the iron-based shape-memory alloy. However, the prior art has the following problems: In the prior art, at least one element of chromium, nickel, cobalt and molybdenum is added to improve a corrosion resistance of the alloy as described above. However, particularly because manganese is added in a large quantity as from 20 to 40 wt. % in the prior art, the improvement of corrosion resistance is not necessarily sufficient. Furthermore, the alloy of the prior art, which contains from 20 to 40 wt. % manganese and in addition chromium, tends to form a very brittle intermetallic compound (hereinafter referred to as the ".delta.-phase") because of the presence of chromium. Formation and presence of this .delta.-phase cause serious deterioration of the shape-memory property, the workability and the toughness of the iron based shape-memory alloy.
In view of the circumstances described above, there is a strong demand for development of an iron-based shape-memory alloy excellent in a shape-memory property and a corrosion resistance, but such an iron-based shape-memory alloy has not as yet been proposed.